Well completion apparatus



March 25, 1969 s. M. ZANDMER WELL COMPLETION APPARATUS Sheet Filed Oct. 11, 1967 INVENTOR. SDI/5 MYRON ZAMDIKE'R FIG. 3

ATTORNEYS March 25, 1969 s. M. ZANDMER 3,434,537

WELL COMPLETION APPARATUS Filed Oct. 11, 1967 1 NVENTOR. 30 IS MVAWN ZAA'OME'Q United States Patent US. Cl. 166-100 12 Claims ABSTRACT OF THE DISCLOSURE Well completion apparatus of the kind wherein a bore 'hole casing is adapted to be positioned in a bore hole and duct forming devices of acid resistant metal are secured to the casing in alignment with holes in the casing wall. The invention provides for a novel plug construction for temporarily closing the duct forming devices. The plugs are of acid soluble metal, have an annular Zone of reduced radial thickness and are press-fitted into a complementarily shaped opening of the duct forming devices. The external free surface of the plug is covered by a relatively thin metallic cover member pressfitted into a space formed between the wall of the duct forming device and the plug, the cover member protecting the plug from mud attack.

The invention also discloses a novel safety cap construction for temporarily closing the end of the duct forming device adjacent the casing, the safety cap comprising a plurality of narrow passages permitting flow of fluid from the interior of the casing into the duct forming device but preventing entry of liquid or solid matter into the interior of the casing.

SUMMARY OF THE INVENTION This invention generally relates to well completion apparatus of the kind wherein duct-forming elements are secured to a bore hole casing or liner and may be caused to project laterally from the casing to make contact with a producing zone of the formation.

Considered from one aspect, the invention is particularly directed to closures for such duct-forming elements of the kind wherein an acid soluble metal plug temporarily closes an end of a tube, sleeve, conduit or the like duct or passage forming device (hereinafter collectively being referred to as tube) of the duct-forming element and wherein, upon introduction of acid into the tube, the plug is at least partially dissolved or disintegrated in order to become dislodged from, and thus to clear, the respective tube end.

Considered from another aspect, the invention is concerned with flow-restricting constructions for use in connection with such duct-forming elements for temporarily blocking the flow of fluid in one direction through a tube, while permitting limited flow of fluid into the tube in the opposite direction, the flow-restricting means being made of acid-soluble metal to permit disintegration of the flow-restricting means by acid.

Until recently, bore hole casings or liners have been set in bore holes by a cementing process in which a cement slurry is forced down through the casing and then upwardly around the outside of the casing to fill the annular space between the exterior casing surface and the surrounding wall of the formation. After solidification of the cement, communication between the casing and the producing zone was established by explosive perforation of the casing, e.g. by means of bullets or shaped charges which also penetrated the hardened cement to form passageways or ducts therethrough. This procedure is unsatisfactory as the bullets or charges tend to crack the cement around the passageways, thereby causing ver- "ice tical communication, to wit, up and down movement of fluid around the casing from oneperforation to another. This, in turn, prevents later selective treatment through each perforation to the formation at the end of each duct or perforation, since injected treating material could travel up or down through cracked cement without permitting selective control at the injected places, i.e. the stratum of the formation at the end of each duct.

More recently, an improved method and device for establishing communication between the casing and the producing zone has been suggested. According to this suggestion, a plurality of duct-forming devices are welded or otherwise secured to the outside of the casing in alignment with holes machined into the casing Wall. These duct-forming devices comprise telescoping tubes or sleeves which are in a retracted position during the positioning of the casing in the bore hole. When contact with a producing zone is to be made, these telescoping tubes are caused to project substantially horizontally toward the formation wall to make contact with the pay zone and to establish a permanent link between the pay zone and the casing. The cement slurry is introduced into the space between the casing and the formation wall either before or after the lateral telescoping of the tubes so that the cement sets around the tubes and the casing. The telescoping tubes of the duct-forming devices, as previously proposed, are made of steel or the like acid resistant metal and the outer tube ends which ultimately contact the producing formation are blocked by acid soluble metal screw plugs which are threaded into the tube in a sealing manner so as temporarily to prevent passage of material through the tube. When communication between the pay zone and the interior of the casing is to be established, acid is introduced into the casing and into the laterally extending telescoping tubes to cause dissolution of the plugs.

In the known constructions the acid soluble screw plug has an external thread which meshes with a complementary interior thread of the tube. Experience has demonstrated that such screw plug constructions are disadvantageous from the standpoint of manufacture and assembly. It is therefore an object of this invention to improve on the prior art plug constructions, by providing a plug which is lodged in the tube by mere pressure or friction fit.

The plug, during lowering of the equipment into the bore hole and before the plug is disintegrated by the acid, is subjected to chemical and mechanical attack from the exterior formations. In particular, caustics of extremely high pH amounting to values up to 14 are contained in the mud surrounding the bore hole which mud exerts a premature disintegrating effect on the plug. Accordingly, it is a further feature of the invention to protect the plug from outside attack by providing a cover member which at least partially embraces the exterior surface of the plug. In a preferred embodiment of the invention, the cover member is of a different metal than the metal of which the plug is composed.

Extensive experiments and tests have established that, irrespective of the strength and the nature of the acid introduced into the tubes, the plugs do not readily and rapidly dissolve unless proper circulation is ultimately provided for the acid. This is so because acid which attacks a surface of a solid acid soluble material dissolves first the outermost layer or stratum of the material whereafter the acid is prevented from effectively exerting its dissolving activity due to the dissolution or reaction product or products which have a tendency to form a protective barrier or cushion between the surface to be dissolved and the acid. For this reason, fresh or unconsumed acid is prevented from contacting the subjacent stratum of acid soluble material. This phenomenon is particularly pronounced in respect to acid attacking relatively small areas. Thus, for example, if one surface of a magnesium plate or disc is wetted with acetic acid, the acid almost instantaneously dissolves a thin surface layer of the wetted surface, whereafter, however, the acid is rendered substantially inactive due to the formation of magnesium acetate which tends to lodge itself as a barrier between the surface to be attacked and unconsumed acid, thereby preventing further effective attack of the unconsumed acid on the undissolved material. For this reason, it has been found that an excessive time is required to dissolve the metal plug in the tube referred to so as to cause dislodgement thereof from the tube.

Accordingly, it is a further object of this invention to provide for a closure construction wherein the end of the acid resistant metal tube of the duct-forming element which upon projection is intended to make contact with the producing formation is temporarily blocked by an acid soluble metal plug of a construction that overcomes the prior art drawbacks referred to and that permits effective dissolution of the plug by the acid and ensures sufficient ultimate circulation and movement of the acid during the dissolution procedure so as to cause disintegration and dislodgement of the plug within reasonable time limits.

In the previously suggested well completion apparatus with which this invention is concerned, the duct-forming devices are welded or otherwise secured to the outside of the bore hole casing in alignment with holes machined into the casing wall. When contact with the producing zone is to be made, the telescoping tubes are projected from their retracted position to extend laterally from the exterior of the casing toward the formation wall. This is accomplished by pressurizing the casing. Since the rear end of the tube of each duct-forming device, to wit, the end which is in alignment with the respective hole in the casing wall, communicates with the interior of the casing while the front end of the tube, to wit, the end which is intended to contact the producing formation, is closed by the plug, pressure builds up within the interior of the tube. This pressure must be sufficient to overcome the retaining force of retaining means or the like to propel the telescoping tubes in a direction away from the casing and toward the producing formation. In practice, it has been experienced that due to malfunctioning or other mishaps as, for example, faulty assembly of the duct-forming devices, some of the tubes are sometimes prematurely propelled toward the producing formation. This, for example, may happen during the lowering of the casing into the bore hole. If a tube has thus been prematurely projected away from the casing, the front portion of the tube which contains the terminal plug may be torn off by frictional contact with the formation wall with the result that free communication is established between the interior of the casing and the space surrounding the casing. This, of course, is highly undesired since it prevents effective build-up of pressure within the casing and may cause reentry of liquid cement from the outside into the interior of the casing.

Accordingly, it is a further object of this invention to provide a construction wherein the rear end of each ductforming device is fitted with a flow-restricting safety means which permits restricted flow of fluid from the interior of the casing toward the terminal plug, but which prevents flow of liquid through the tube into the interior of the casing.

A still further object of the invention is that the flow restricting safety means comprises a cap provided with a plurality of relatively narrow bores or passages transversing the cap, the size of the passages being sufficiently large so as to permit flow of pressurizing fluid from the casing toward and into the tube, but being small enough so as to block the passage of liquid or solid matter, such as cement slurry.

A still further object of the invention is to provide a safety device of the indicated kind which, after positioning of the casing with its duct-forming devices in the bore hole and after projection of the tubes toward the producing formation, can at least partially be disintegrated or dissolved by acid so as to permit substantially unrestricted flow through the tubes in both directions.

A still further object of the invention is to provide a safety device of the indicated kind which is in the form of a screw cap and which is constructed so that after the flow-restricting means have been dissolved by acid, a seat surface is exposed on the cap at the location of the associated casing hole to permit blocking of the communication between the interior of the casing and the tube of the duct-forming device by means of a ball-shaped sealing means, commonly referred to as a ball sealer, which is suspended within liquid contained within the casing and thus has a tendency to be urged against said seat when liquid flows from the interior of the casing through the hole.

Generally, it is an object of this invention to improve on the art of well completion apparatus having ductforming elements as presently practiced.

Briefly, and in accordance with the first indicated aspect of this invention, the acid soluble metal plug is constructed in the form of a generally cylindrical body pressfitted into a complementary receiving opening of the tube. The body is formed with an interior bore or cavity which extends inwardly from the rear end of the plug toward the front end, but which terminates short of the front end to leave a front portion of substantial metal layer thickness. The peripheral wall of the body which defines said bore or cavity has an annular zone of substantially reduced radial thickness which forms a diaphragm-like strip. When the plug is to be dissolved, acid is introduced into the rear end of the plug. The acid enters the bore or cavity of the plug and rapidly dissolves the metal in the region of the annular zone of reduced radial metal thickness, to wit, the diaphragm strip. The acid then exits outwardly through the opening in the diaphragm zone thus formed and flows along the external surfaces of the plug to cause rapid dissolution and disintegration of the plug.

According to a further feature of the invention, no chemical sealant is used for sealing the plug against the wall of the tube, but the sealing is advantageously accomplished by an O-ring which is positioned in a groove located adjacent the rear end of the plug. As is known, O-rings are extremely efiicient sealing means and in fact the sealing action of an O-ring is the greater the greater the pressure. Therefore, the provision of the O-ring adjacent the rear end of the plug effectively prevents entry of matter into the tube from the outside. The O-ring moreover blocks escape of material from the tube interior toward the outside. The diaphragm strip which is thus first dissolved and eaten through by the acid should be located ahead of the O-ring, to wit, between the O-ring and the front end of the plug so that the flow of the acid is not obstructed.

The annular diaphragm zone of substantially reduced radial thickness is preferably formed by cutting or working a relatively narrow groove into the interior body wall which defines the bore referred to. This groove should have a depth so that the remaining wall portion is extremely thin but yet of a strength suflicient to withstand the pressures acting on the plug. Since the diaphragm strip portion is protected from erosion and other forces which may prematurely cause its destruction, the diaphragm may be extremely thin. In practice, it-has been established that at bore hole depth of 15,000' feet for which duct-forming devices of the indicated kind are presently designed, excellent results are obtained if the thickness of the diaphragm strip is about between 0.030 to 0.040 inch.

After the acid has eaten through the diaphragm strip the plug becomes dislodged from its friction fit engagement with the tube. In accordance with a further feature of the invention, the tube is provided with retarding or retaining means which are located behind the rear end of the plug and which advantageously may comprise a flange or shoulder projecting from the interior wall of the tube in an inward direction so as to provide a seat for the plug against which the plug rests and which prevents inward movement of the plug. Thus any tendency to urge the plug inwardly is effectively counteracted. Only after substantially the entire plug has been consumed by the acid will plug fragments be able to clear the shoulder to enter the interior of the tube.

The protective cover member is press-fitted onto the plug by providing a suitable space between the plug exterior and the tube wall, the cover member having a flange which is forced into the space.

Turning now to the second aspect of this invention, the flow-restricting means advantageously takes the form of a safety cap which is screwed or otherwise secured to the rear end of the duct-forming device at a location substantially flush with the interior wall of the casing and coinciding with its associated hole. A narrow passage is formed through cap, conveniently in the center thereof, by drilling a hole of small diameter through the cap surface. Experience has demonstrated that the effect is considerably enhanced if a plurality of additional holes of smaller diameter than that of the hole forming the narrow central passage are provided at spaced locations of the cap surface. Tests have indicated that excellent results are obtained if the central hole has a diameter of about 0.040 inch and if three smaller holes each of a diameter of about 0.021 inch are placed in spaced relation from the central hole. The size of the holes is sufiicient to permit the build-up of pressure within the duct-forming device but is small enough to prevent passage of cement slurry or other liquids or solids in either direction. In view of the smallness of the holes, it is generally not required to provide valve means in the holes. However, if desired, a check valve may be arranged on the cap adjacent the central passage to prevent flow of fluid from the duct-forming device through the central passage into the interior of the casing. The check valve may then preferably be of the ball check valve type.

While it is feasible to make the entire cap of acidsoluble metal, such as zinc or magnesium, according to one embodiment, the cap proper is made of steel in the same manner as the duct-forming device proper, the cap, however, having a central, circular insert of acid-soluble metal, such as zinc or magnesium. The holes are then located in the acid-soluble insert and after the telescoping tubes have been projected into their operative position by pressurizing the casing, whereby fluid flow through the passage or holes in the insert causes projection of the tube, acid is inserted into the casing to dissolve and integrate the insert. The fluid which is used for pressurizing the casing may contain weak acid. The weak acid requires, however, a relatively long time to dissolve the insert, so that sufiicient time is available properly to eject the telescoping tubes. The dissolution of the insert by the acid is, of course, facilitated by the provision of the passages through the insert since this enlarges the contact surface available to the acid for attack and increased circulation. It has been found that the acid dissolving effect can be considerably enhanced by providing an uneven or rough cap surface which of course further increases the surface area subject to acid attack. It has also been ascertained that excellent results are obtained if the acid resistant body portion of the cap is covered by a suitable coating material such as, for example, Teflon. After the insert has been destroyed by the acid, substantially unrestricted communication between the interior of the casing and the duct-forming device is thus established, since the remainder of the cap then defines a relatively large passageway.

According to a preferred embodiment of the invention, the acid-soluble insert of the cap forms an inwardly bevelled or conical surface which bears against a complementarily shaped surface of the acid resistant portion of the safety cap. After the acid-soluble insert has been dissolved by the acid, there thus remains the acid resistant cap portion which defines a passageway having an interior diameter corresponding to the diameter of the acidsoluble insert and forming a conical peripheral seat surface. If this enlarged passage or passageway, during the operation of the well, is to be closed for any reason Whatsoever, a ball sealer can be floated down and suspended within liquid in the casing to lodge itself against the conical surface in a sealing manner. The diameter of the acidsoluble insert and thus of the passageway formed upon dissolution should, however, be smaller than the diameter of the telescoping tubes to prevent undesired retraction or entry of the tubes into the interior of the casing should they in any manner become dislodged from their retainmg means.

The various features of novelty which characterize the invention are pointed out with particularly in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings in which there is illustrated and described a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a fragmentary side elevation of a bore hole casing having a duct-forming device secured to its exterior surface, the duct-forming device including a terminal tube or sleeve which is closed by an acid-soluble metal plug in accordance with this invention. In order not to crowd the drawing, those elements and mechanisms of the duct-forming device which do not form part of this invention have been omitted;

FIG. 2 is a sectional view along line 2-2 of FIG. 1 on an enlarged scale;

FIG. 3 is a sectional view along line 3-3 of FIG. 2;

FIG. 4 is a fragmentary elevational view of a different embodiment of safety cap having a rough surface;

FIG. 5 is a fragmentary view of still another embodiment of a safety cap having an .acid insoluble insert and an acid resistant body portion;

FIG. 6 is a sectional view along line 66 of FIG. 5; and

FIG. 7 illustrates the terminal plug with its cover member as press-fitted into a tube as shown in FIG. 1.

Referring now to the drawings, and in particular to FIGS. 1 and 2, a duct-forming device, generally indicated by reference numeral 1.00, is secured to the external sur face of a bore hole casing 10. For this purpose, the bore hole casing 10, which has a machined hole or opening 99 as seen in FIG. 2, is provided with a nipple or mounting boss 08 which is welded to the exterior casing surface as indicated by reference numeral 97. The duct-forming device 100 has a hollow, cylindrical rear mounting portion 96, whose external thread meshes with the internal thread 94 of the nipple 98. Rear mounting portion 96 of the duct-forming element we terminates in a front collar or flange portion 93 which fits into an annular space 92 defined by the shoulder 91 formed by the nipple 98. A sealing means, such as an O-ring 66, may be inserted between the collar 93 and the shoulder surface 91 of the nipple 98, in order properly to seal the duct-forming element against the nipple 98. For this purpose, the collar 93 is provided with an annular groove 89 as seen in FIG. 2. The general construction of the duct-forming device has not been illustrated, but may correspond to that disclosed in US. Patent No. 3,245,472.

The duct-forming device 100 projects laterally from the casing 10 and includes an outer or terminal tube or sleeve 13 into which is friction fitted a plug generally indicated by reference numeral 50.

The construction of the plug is illustrated in detail in FIG. 7. It will be noted that the plug 50 comprises a body portion 14. An external peripheral groove 16 is cut into the plug adjacent its rear end 17 and an O-ring 18 of customary construction is lodged within the groove.

The interior of the body portion 14 is hollowed out to form an interior cavity or bore 19 which extends from the rear end 17 towards the front end 20 but terminates short thereof. The bore 19 should not extend too far towards the front since the thickness of the front end 20 must be sufficient so as to be able to withstand the pressures which act on the free surface of the front end 20 from the outside. The thickness of the front end 20 of the plug 50 should also be relatively substantial since the front surface of the plug, during the lowering of the casing to near the bottom of the well bore, will consistently rub against the formation wall so that it must be able to withstand substantial impact, abrasion and friction forces. In accordance with the invention, an internal groove 21 is formed in the body portion 14 which grove 21 communicates with the bore or cavity 19 and forms a body wall portion 22 of substantially reduced radial diameter. Wall portion 22 of the plug is thus weakened and constitutes in fact a diaphragm-like strip. The diaphragm 22 is subject to rapid attack and destruction by acid. From a practical point of view, experiments have demonstrated that for plugs of the indicated kind to be used in duct-forming devices at well depth of about 15,000 feet, the thickness of the zone 22 or diaphragm should be between about 0.030 and 0.040".

FIG. 7 shows the plug 50 as being friction or pressfitted into a tube such as the tube 13 of FIG. 1. For this purpose, tube 13, near its free end portion 13', defines an opening 110 of a diameter slightly smaller than that of the plug 50 so as to permit friction fitting of the latter With the interposition of the O-ring 18. Due to the provision of the O-ring 18 lodged within the groove 16 of the plug, effective sealing between the plug 50 and the tube 13 takes place, thereby preventing entry of matter from the outside into the interior of the tube and at the same time preventing penetration of matter from the interior of the tube toward the outside. It will be noted that the tube 13 forms an annular shoulder portion 26 which provides a seat for the rear end 17 of the plug. The opening 110 is widened at the end portion 13' of the tube to provide a space or clearance 111 between the wall of the tube 13 and the exterior of the body portion 14 of the plug 50 to accommodate, by press fit, a metallic cover member 112 which protects the plug portion 20 from chemical and mechanical attack. The cover member 112 is relatively thin and should preferably be of a different metal than the metal of which the plug 50 is composed. In practice it has been ascertained that excellent results are obtained if the cover member is made of zinc while the plug proper is made of magnesium. Zinc has a superior resistance to the caustics contained in mud and is therefore particularly suitable for the intended purpose.

It will be noted that the hollow threaded rear mounting portion 96, which is screwed into the nipple 98, has an exteriorly threaded end portion 85. End portion 85 extends into the hole 99 formed in the casing wall 10. In accordance with the second aspect of the invention, a safety cap, generally indicated by reference numeral 80, has an internal thread 79 on a flange portion 49 which meshes with the external thread 78 of the end portion 85. The safety cap 80 may thus be screwed onto the end portion 85 and removed therefrom at will. FIG. 2 shows the cap in the operative position in which it is screwed onto the end portion 85. It will be noted that cap 80 is provided with a central hole or passage 74 and three narrower passages or holes 114, 115 and 116. The wider central hole or passage '74 has preferably a diameter of about 0.040 inch, while the diameter of each of the three smaller holes is about 0.021 inch. Although a total of four holes or passages have been shown, a different number may be adopted. The cap 80 of FIGS. 2 and 3 is of acid soluble material. Upon introduction of acid into the interior of the casing, the cap thu's dissolves.

According to the embodiment of FIG. 4, a cap generally indicated by reference numeral is shown which is also provided with four passages 114, 115', 116 and 174. The difference between the cap 80 of FIGS. 2 and 3 and the cap 1850f FIG. 4 is that the latter has a rough surface 181 which facilitates the solution by acid.

Referring now to the embodiment of FIGS. 5 and 6, a cap structure generally indicated by reference numeral is shown which has an acid resistant body portion 191 and an acid soluble insert portion 192. This acid soluble insert portion, made of zinc, magnesium, aluminum or the like, is provided with a central passage 193 essentially corresponding to the passages 17 4 and 74 of the other embodiments and three smaller passages 194, 195 and 196. The acid resistant body portion 191 may be covered by a suitable chemical coating material such as, for example, Teflon, as indicated in FIG. 5 by reference numeral 196. The acid resistant body portion 191 of the cap forms a conical or beveled surface 68 against which bears the complementarily shaped conical or beveled surface 67 of the acid soluble insert 192.

In practice, the acid resistant portion 191 of the cap is first manufactured with a passageway of a diameter slightly smaller than the diameter of the acid soluble insert 192, whereafter the acid soluble insert is press-fitted into the passage.

The operation of the device is as follows:

In respect to the safety cap After the casing 10 has been positioned within the bore hole and a cement slurry or the like has been pumped through the lower open end of the casing to envelop the casing and to fill the space between the exterior of the casing and the wall of the formation, the pressure within the casing is increased to a value so as to overcome the tube retaining means and to propel the telescoping tubes of the duct-forming devices through the cement slurry and toward the wall of the formation to make contact therewith. The fluid which is used for this purpose flows through the passages in the cap and the retaining force of the retaining means which has prevented the projection of the telescoping tubes is overcome by the fluid pressure. However, in the event that the portion of the duct-forming device 100 extending beyond the collar 93, including the screw plug 50, should have prematurely been ejected and been torn or broken off due to faulty assembly or the like, entry of any material into the interior of the tube is prevented by the cap, the passages being too small to permit entry of liquids into the casing interior.

Further, even if the front portions of one or several duct-forming devices are broken oil, pressure can still be built up within the casing to a suflicient value to eject the remaining devices. This is so because leakage through the narrow passages of the broken off elements can be easily compensated for by increasing the pumping speed. By contrast, in the prior art constructions without safety cap, the breaking off of a single duct-forming element resulted in a relatively large passageway from the casing to the exterior which seriously interfered with the operation.

Once the casing has been installed and the duct-forming tubes have been properly propelled into contact with the wall of the formation, acid is introduced into the easing to eat away and dissolve the acid-soluble insert or cap proper, whereby a relatively large passageway is formed. If, at a later stage during operation of the well, communication between the interior of the casing and the duct-forming device is again to be blocked, a ball sealer can be floated down the casing interior, whereby the ball sealer will be pressed against the conical surface 67 of the acid resistant portion 191. of the cap to again block the communication.

In respect to the acid-soluble plug After the tube 13 with the plug 50 and the cover member 112 press-fitted thereinto has been moved from its retracted position within the casing to the extended or projected position shown in FIG. 1, and the cap has been disintegrated by the acid, the acid enters the tube 13. If the plug 50 is made of magnesium or zinc, acetic acid will normally be used or other organic or mineral acids may be employed. The acid thus enters the bore or cavity 19 of the plug and attacks the Wall defining the cavity. Due to the provision of the groove 21 defined by the thin diaphragm wall portion 22, the diaphragm 22 is rapidly eaten through by the acid. The formation of the groove and its particular shape facilitate the acid attack on that region due to local accumulation of acid which speeds up the dissolution of the diaphragm zone. The acid, after having caused disintegration of the wall portion 22, flows then along the external surface of the plug. Since no chemical sealant is introduced between the plug and the tube wall, the flow of the acid is unobstructed and rapid disintegration of the external surface of the plug 50 is thus caused. The acid also disintegration the cover member 112 which usually is at least partially destroyed by the chemical action of the surrounding formation. The external pressure acting on the front end of the plug may then urge the plug 50 towards the interior of the tube if this pressure exceeds the pressure within the tube. Due to the provisions of the shoulder 26, dislodgement of the plug is, however, prevented. The presence of the O-ring 18 prevents the acid from reaching the rear end of the plug so that the front portion of the plug and the cover member are consumed before the acid eats around the O-ring. Therefore, no large fragments of the plug are hurled towards the interior of the tube 13, but only small pieces are flushed inwardly which may be formed by the disintegration of the plug.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. In well completion apparatus of the kind wherein a bore 'hole casing is adapted to be positioned in a bore hole and duct-forming devices of acid-resistant metal are secured to the casing in alignment with holes machined in the casing wall, each duct-forming device being capable of being laterally extended from the casing for making contact with a producing formation and wherein each duct-forming device includes a terminal sleeve whose outer end is blocked and closed by an acid-soluble metal plug which upon introduction of acid into the sleeve is at least partially dissolved to become dislodged from its associated sleeve, the improvement which comprises means on said sleeve and said plug permitting the acid soluble metal plug to be press-fitted into an opening of said sleeve, and a cover member mounted on said plug to protect the exterior surface of the plug, said cover member being of acid-soluble material.

2. The improvement of claim 1, wherein said plug and said sleeve define an annular space, said cover member having a flange portion friction-fitted into said space.

3. The improvement as claimed in claim 1, wherein said acid soluble plug includes an annular zone of substantially reduced radial thickness.

4. The improvement as claimed in claim 1, wherein sealing means are inserted between said plug and the associated sleeve adjacent the rear end of said plug, said sealing means being in the form of O-ring means.

5. The improvement as claimed in claim 1, wherein said cover member is made of metal, the metal of the cover member being different than the metal of the acid soluble metal plug.

6. The improvement as claimed in claim 5, wherein the cover member is made of zinc and the acid soluble plug is made of magnesium.

7. The improvement of claim 1, wherein a safety cap is secured to the end of the duct-forming device which is closest to the casing, said safety cap being substantially in alignment with the wall of the casing and located substantially within said machined casing hole and having a plurality of narrow passages which permit flow of fluid from the interior of the casing toward said plug but prevent How of liquid through the sleeve and into the interior of the casing, said safety cap at least partially being made of an acid soluble metal.

8. The improvement as claimed in claim 7, wherein the surface of said cap facing the interior of the casing is rough.

9. The improvement as claimed in claim 7, wherein said plurality of passages includes a central passage and a plurality of passages spaced from said central passage, said central passage being larger than said spaced pas sages.

10. The improvement as claimed in claim 9, wherein said central passage has a diameter not exceeding about 0.040 inch, said smaller passages having a diameter not exceeding about 0.021 inch.

11. The improvement as claimed in claim 7, wherein said cap has a body portion of acid resistant metal and a central portion of acid soluble metal, said central portion containing said passages.

12. The improvement as claimed in claim 11, wherein said acid resistant portion is covered by Teflon.

References Cited UNITED STATES PATENTS 2,708,000 5/1955 Zandmer 166l00 2,780,293 2/1957 Zandmer 166100 2,855,049 10/1958 Zandmer 166100 3,120,268 2/1964 Caldwell 166-100 3,326,291 6/1967 Zandmer l66100 3,382,926 5/1968 Zandmer 166-100 DAVID H. BROWN, Primary Examiner.

US. Cl. X.R. 166-296 

